1. Technical Field
The present invention relates to an automatic focusing system, and more particularly, to an automatic focusing system that includes an image pickup device dedicated to an automatic focusing (AF) operation and that automatically adjusts the focus of a photographing optical system by the use of a contrast of a subject image input by the image pickup device.
2. Related Art
In an automatic focusing function employed in video cameras such as television cameras, a contrast method is generally used in which the contrast of a subject image is detected from a video signal obtained by an image pickup device and the focus is controlled to be the largest (maximum). The contrast of the subject image is quantitatively detected by extracting high-frequency components from a video signal (brightness signal) obtained by the image pickup device and adding the signals of the high-frequency components in the unit of field. The added value represents the magnitude of the contrast of the subject image and also represents a degree of focusing. The added value is referred to as an estimated focus value in this specification.
It has been suggested that an AF-dedicated image pickup device (hereinafter, referred to as AF pickup device) for taking an AF subject image is provided independently of an image pickup device (hereinafter, referred to as video pickup device) for taking an original image as a video camera itself. For example, beam splitting means such as a half mirror is disposed in a photographing optical system that forms a subject image on the video pickup device and the subject beam input to the photographing optical system is branched into two optical paths. The subject beam branched into one optical path (video optical path) is incident on the video pickup device to form a subject image on the pickup face of the video pickup device. The AF pickup device is disposed in the other optical path (AF optical path) and the subject beam branched into the AF optical path is incident on the AF pickup device to form a subject image on the pickup face of the AF pickup device. The optical system disposed in the AF optical path is configured so that the subject image formed by the AF pickup device is equal to the subject image formed by the video pickup device. The focus of the optical system for the video pickup device is properly adjusted by adjusting the focus of the photographing optical system so that the contrast of the subject image obtained by the AF pickup device is the highest.
The exposure dose adjustment (Quantity of light adjustment) for the AF pickup device is performed by a mechanical iris diaphragm and an electronic shutter function used in common in the quantity of light adjustment of the video pickup device. The mechanical iris diaphragm adjusts the quantity of light adjustment of the video pickup device to be appropriate, but the electronic shutter adjusts the quantity of light independently of the video pickup device so that the exposure dose of the AF pickup device is appropriate.
The subject image (video) acquired by the AU pickup device is not used as a recording or reproducing image like the subject image (video) acquired by the video pickup device. Accordingly, it is possible to freely set a frame rate (image input period) at the time of allowing the AF pickup device to input the subject image (video). On the other hand, when the frame rate of the AF pickup device is set to be high (when the image input period is set to be low), it is possible to reduce the detection period of the contrast of the subject image, that is, the detection period of the estimated focus value. When the detection period of the estimated focus value is reduced, the automatic focusing operation having excellent subject following ability can be performed. Therefore, it is preferable that the frame rate of the AF pickup device is set to be as high as possible.
JP-A-2004-212458 (Corresponding to US 2004-0130652) Patent Document 1 discloses that the input of the AF subject image when a shutter button at the time of photographing a still image is halfway pushed is performed at a frame rate higher than the input of the video-photographing subject image when the shutter button is fully pushed.
However, Patent Document 1 does not disclose a change in frame rate at the time of continuously performing the input of the AF subject image and an improvement in AF following ability for a moving subject.
The electronic shutter speed (exposure time) varies depending on the brightness of a subject or the like and the frame rate of the AF pickup device cannot be set to be higher than the electronic shutter speed. Accordingly, since the frame rate of the AF pickup device cannot be kept to set to a high constant speed, the frame rate need be set in consideration of a case where the electronic shutter speed is low. Therefore, it is not possible to simply enhance the frame rate of the AF pickup device.
The invention is contrived in consideration of the above-mentioned situations. An object of the invention is to provide an automatic focusing system which has an AF pickup device dedicated to an AF operation and which can perform the AF operation with high following ability to a moving subject.